PROCEEDINGS, 41st Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 22-24, 2016 SGP-TR-209 1 Study of Prevention and Mitigation of Stuck Pipe in Geothermal Drilling Bonar MARBUN, Arif SOMAWIJAYA, Arnold R. NOVRIANTO, Haniyyah HASNA, M. Ridha ANSHARI Institut Teknologi Bandung Jalan Ganesha 10 Bandung, West Java, 40132, Indonesia e-mail: [email protected]Keywords: geothermal drilling, stuck pipe, drill string, casing. ABSTRACT Stuck pipe of geothermal well drilling occurs when the pipe, for the likes of drill string and casing in the wellbore cannot be pulled out without exceeding its designed working load. This incident will result non-productive time and unnecessary additional cost. Historically, one out of three wells drilled in geothermal area in Indonesia experienced stuck pipe problems. Most cases of stuck pipe of geothermal well drilling in Indonesia can be avoided with good drilling design and operation. This paper shows the stuck pipe incidents from a plenty of geothermal wells throughout the Indonesian geothermal fields. The data of design and the execution of wellbore where the stuck pipe incidents have occur will be presented and explained. The analysis and evaluation based on the wellbore data will be carried out. The method of stuck pipe causes, prevention and mitigation will be build. 1. INTRODUCTION Geothermal systems are widely produced from a hydrothermal system. Hydrothermal systems contain water in the pore and fracture inside the rock with large permeability that is enough to produced large volume of fluids. Geothermal formations have the following characteristics which are (1) relatively high temperature from 160 to above 300 C, (2) compressive strength 240 + MPA (compared with the sediment reservoir 100 + MPa), abrasive (quartz content above 50%), highly fractured, under pressured, contain corrosive fluids and formation fluids containing high solid content (Finger and Blankenship, 2010). There are two classifications of rocks, brittle and ductile. A rock is brittle if, when subjected to stress, it breaks without significant deformation (strain). A rock is ductile if, when subjected to stress, it can deform under tensile stress. Rocks that are often encountered in geothermal reservoir are granite, granodiorite, quartzite, greywacke, basalt, rhyolite and volcanic tuff. Geothermal reservoir rock which is brittle and highly fractured could lead to breakouts and results in caving. Caving is not desired during drilling operation because it causes lots of problems such as restricted circulation, borehole enlargement, and stuck pipe. Those problems can be avoided with good drilling design and operation. The term of stuck pipe is when the drill string cannot longer free to move up, down, or rotates as the driller wants. In drilling operation of a geothermal well, stuck pipe is one of the most common problems occurred. Stuck pipe leads to loss of time and can be very costly, including lost drilling time during freeing the pipe, cost of fishing, and even it risks abandonment of tools inside the hole. Concerning these problems and in order to improve the drilling operation performance, evaluation towards stuck pipe incidents in geothermal drilling is performed. Certain parameter are taken into account in handling stuck pipe and in order to perform a free-stuck- pipe drilling operation, such as rock strength (compressive and tensile), and cuttings information from daily mud report. This paper will focus in analising the wellbore stability to ensure that the drilling operation in geothermal reservoir can be conducted safely and effectively to avoid drilling problem and non-productive time. 2. METHODOLOGY Geothermal well instability is affected by lots of factor. Several factors which will be discussed in this paper are the mineralogy, clay support, natural fractures, and permeable formation. The mineralogy and clay support trigger the chemical activity in the formation which result in shale swelling. The shale swelling leads to tight hole problem which can be observed from Gumbo accumulation on drill string and small caliper log value. The high productivity of geothermal system have characteristics of high porosity (natural fracture) and high permeability. The closely space and weak planes (based on natural fracture characteristic) lead to existing fracture enlargement and large caving. Meanwhile, high permeability causes lost circulation problem which can lead to inadequate hole cleaning and accumulation of cutting bed. On the other hand Fracture enlargement leads to borehole enlargement which can be observed from UBI and caliper log. Large caving and lost circulation can be detected from surface parameter resulted by restricted annulus. The indicators of restricted annulus are low weight- on-bit (DWOB), low rate of penetration (ROP), restricted circulation, and pump pressure spike. The methodology which is used to identify the wellbore instability mechanism is shown in Figure 1
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Study of Prevention and Mitigation of Stuck Pipe in Geothermal Drilling
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PROCEEDINGS, 41st Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, February 22-24, 2016
SGP-TR-209
1
Study of Prevention and Mitigation of Stuck Pipe in Geothermal Drilling
Bonar MARBUN, Arif SOMAWIJAYA, Arnold R. NOVRIANTO, Haniyyah HASNA, M. Ridha ANSHARI
According to criteria compressive shear failure that is combined with rock strength concept, stress value which can lead to breakout can
be predicted. On the other hand, mud weight at tensional fracture or hydraulics can be estimated from tensile strength approximation.
Mud weight calculation is done based on geomechanics model where the main stress direction at the wellbore are taken into account.
Equation that is used for mud weight calculation is:
(1)
where 𝑃𝑤,min, 𝑃𝑓, 𝜎𝑣, |𝑣𝑓𝑟|, 𝜎𝐻, 𝜎ℎ, 𝑃𝑓, 𝐶𝑜, 𝛽 are lower mud-weight value, pore pressure, absolute value of Poisson’s ratio, maximum
horizontal stress, minimum horizontal stress, unconfined compressive strength, and rupture angle of rocks respectively.
Wellbore instability indication can be seen through stuck pipe as the lost drilling time. Causes that can lead to wellbore instability that
will be discussed further are breakouts and closely spaced natural fracture/weak planes.
3. FRACTURED ROCKS
More than a half of all known geothermal resources occur in fractured igneous formations (Sanyal, et al, 1979). Most commercial
hydrothermal resources exist as dual porosity system. The high productivity of geothermal system is related to the highly fractured and
abrasive rocks. The highly productive fracture system is required to make geothermal projects economically viable, but is also the basis
of the endemic stuck pipe problems encountered when drilling geothermal wells. (Finger and Blankenship, 2010)
From what we have discussed before, fractured igneous rocks have a brittle and abrasive characteristics means the rocks have a high
tendency to fall into pieces and are very loose. The rocks can fall into the wellbore and jam the string in the hole (Nguyen et al., 2009).
Inadequate hole cleaning will also causing the rocks to pack-off
Figure 7. Pack-off mechanisms in fractured or mechanically incompetent rock (Rabia, Well Engineering and Construction)
When the well packs off, there is a sudden reduction or loss of the ability to circulate, and decrease pump pressures follow, which
indicates the pipe is stuck caused by pack-off. Pack-off will also result squarish and high volume of cuttings in shale shaker. Pack-off
causes borehole enlargement due to high tendency of the rocks to fall into pieces. Due to this borehole enlargement, the weight-on-bit
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(DWOB) readings at surface will be decreased, and by small cuttings jamming under the bit, the rate of penetration (ROP) will also
decrease while the torque will increase. From Logging While Drilling, gamma ray will show high value (Gamma Ray API > 60) and
resistivity follows the fracture and bedding plane orientation. This event indicates pack-off during drilling. The inability to circulate due
to pack-off is indicated by higher hookload due to the decrease of buoyancy force acting on the drill pipe.
Table 1. Wellbore Instability indicator due to fractured rocks at borehole condition
Table 2. Wellbore instability indicator due to fractured rocks at surface condition
3.1. Prevention of Stuck Pipe Due to Fractured Rocks
1. Minimize drill string vibration during drilling operation.
2. Choose an alternative revolution per minute (RPM), lower preferably, or change the BHA configuration if high shock vibrations are
observed.
2. Do proper hole cleaning practice (reaming, circulate before tripping out).
3. Slower tripping speed, especially before entering the fractured zone.
3.2 Mitigation of Stuck Pipe Due to Fractured Rocks
1. As soon as it is suspected there is pack-off (a sudden reduction or loss of the ability to circulate, and high pump pressures and
torques), pump stoke must immediately reduce to half. This minimize the likelihood of pressure trap to occur (the higher the likelihood, the worse the pack-off will get).
2. If the pipe is already stuck, shut down the pump and bleed off the pressure in standpipe (do not do this if there is non-ported float
valve). During the bleed off, flow rate of drilling fluid is maintained to prevent U-tube occur, where solid particles from cuttings
plug the drill pipe.
3. Use minimum unbalance pressure (<500 psi) under the pack-off zone. This will be the indicator when pack-off is managed and the
pressure should be bleed off.
4. Maintain standpipe pressure below 500 psi and give torque (just enough to be able to connect the drill string and joint). Do not move
the drill pipe up and down.
5. Continue giving torque and monitor the bleed off pressure and flow circulation in shale shaker. If there is back flow to the surface,
increase the pump stroke gradually to maintain standpipe pressure at 500 psi. If there is still back flow to the surface, increase the
pump stroke immediately.
6. If there is no back flow to the surface, try to free the drill pipe by moving (at free points of drill pipe) up and down. Do not give
excessive force to the drill pipe. During the effort to free the drill pipe, maintain the standpipe pressure at 500 psi.
7. Do not do jarring. If the back flow still occurring, increase the stand pipe pressure to 1500 psi and continue to free the drill pipe
while keep giving torque.
8. If full back flow to the surface occurs, then it is allowed to do jarring. If pack off happened during tripping out, do jarring
downwards. If pack off happened during the drill pipe is moving down, do jarring upwards.
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4. LOST CIRCULATION
The most expensive problem routinely encountered in geothermal drilling is lost circulation, which is the loss of drilling fluid to pores
or fractures in the rock formations being drilled. Lost circulation represents an average of 10% of total well costs in mature geothermal
areas and often accounts for more than 20% of the costs in exploratory wells and developing fields. Well costs, in turn, represent 35-
50% of the total capital costs of a typical geothermal project; therefore, roughly 3.5-10% of the total costs of a geothermal project can
be attributable to lost circulation.
Figure 8. Lost circulation (Rabia, Well Engineering and Construction)
One of the best solution for loss circulation with if an adequate water supply is available, it is practical to drill without returns. If fresh
water is not available, produced brine, which would normally be reinjected, can be used for drilling wells within a developed project.
Drilling without returns is frequently used when core drilling, where the cuttings are very fine and where much of the rock comes out of
the hole in the form of core. There have been many rotary drilled holes where intervals of many hundreds of meters have been drilled
with complete lost circulation. There are special techniques required to prevent formation collapse and to keep from getting stuck. The
highest risk is when only partial returns are obtained, as the low annular velocities above the loss zones may not be adequate to clean the
hole. High viscosity sweeps are usually used to reduce this risk. Once total loss is encountered, pumping water at high rates down the
annulus as well as down the drill pipe will flush the cuttings away from the wellbore, preventing any sticking problems, and provide
positive wellbore pressure to hold up weak formations.
Conventional lost circulation material is not always successful in the cavernous and vugular low-pressure formations encountered in the
geothermal reservoirs. Based on results from the survey of industry experience, geothermal operators are using conventional petroleum
methods for fighting lost circulation problems.
When lost circulation occurs, it is important that causes be identified in order that proper remedial procedures can be implemented. Lost
circulation causes can be grouped into one of the following four categories:
1. Surface Drilling – for surface hole drilling, formations are generally weak and hole diameter is large. Drill solids from the large hole
can build up, generating relatively high downhole pressure that may break the formation.
2. Rapid Drilling – High equivalent circulating density can cause pressure that may exceed the closure stress. Fast penetrating rates can
load the annulus with drill solids, creating rheology and hydraulic problems.
3. Abnormal Pressure - In drilling through high pressure zones, mud weight is normally increased for well control purposes, but lost
circulation may result if a low or normally pressured zone is also encountered, General practice is to set casing through the high
pressure interval and change back to normal mud weight for deeper drilling. This requires careful casing point selection. Otherwise,
both the high pressure interval and the lost circulation zone will be exposed simultaneously in open hole.
4. Fractured Formations – When encountered, the general practices is to add Lost Circulation Material (LCM) to maintain returns or
use a cement squeeze if LCM is unsuccessful. If necessary, the hole should be forced to desired depth, but the lost circulation
problem must be solved at each interval before drilling deeper since, otherwise, problems can compound or occur later. Casing
should be set as soon as possible and the casing shoe should always be tested. Deeper drilling should not commence unless the
casing seat holds pressure.
The first two categories constitute drilling-induced problems related to hole cleaning capability and equivalent circulating densities.
When drilling is in progress, the presence of these fractures let the drilling fluid enter the formation easily which causes a loss
circulation problem. Loss circulation in drilling operations is the loss of drilling fluid to pores of fractures in the rock formations. This
loss is harmful for several reasons (Marbun et al., 2015):
1. When the drilling fluid is loss to the formation, it will not bring cuttings to the surface and stick the drilling assembly.
2. Loss of drilling fluid to formation will be very costly.
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3. Production zone in geothermal field is usually a loss zone, so it will be difficult to solve the lost circulation problems while preserve
the production ability.
4.1 Prevention of Stuck Pipe Due to Permeable Formation
1. Maintain the correct drilling fluid properties
2. Mix the drilling fluid with fibrous material that will plug the loss apertures in the formation
4.2 Mitigation of Stuck Pipe Due to Permeable Formation
1. Use low density fluid such air/gas, foam, aerated fluid
2. Move drill pipe up and down (at the free point) but do not overpull since it makes the condition worse
3. Do not do jarring during this condition, do the jarring process when full circulation return to the surface
Table 3. Wellbore Instability indicator due to permeable formation at borehole condition
Table 4. Wellbore Instability indicator due to permeable formation at surface condition
5. UNDERGAUGE HOLE
In Geology, interbedding occurs when beds (layer of rock) of a particular lithology lie between or alternate with beds of a different
lithology. (UCMP glossary 2008). Good penetration rates can be expected in interbedded sand and shale formations. However rocks that
are often encountered in geothermal reservoir are granite, granodiorite, quartzite, greywacke, basalt, rhyolite and volcanic tuff (Figure 5
and Figure 6) shows the tensile and compressive strength of rocks which often encountered in geothermal reservoir. This extremely
hard, abrasive, or fractured formations require a bit set with small diamonds and a crowsfoot fluid course to permit a high concentration
of diamonds.
Undergauge hole condition could occur when drilling hard formation (abrasive) zone which result in bit wear and smaller hole diameter
that the bit diameter. This condition could lead to stuck pipe especially at the section which have the same diameter with the bit. When
the wear out bit is pulled out and the new bit is run the drillsting is likely to get stucked, especially if the hole is not reamed when the
drillstring is run in hole. This mechanism could also occur when run in hole the drillstring rapidly without reaming. Usually the coring
tools size are smaller than the bit size, consequently the hole will be smaller and caused the drillstring to stuck. This stuck pipe
mechanism only occurs during tripping in operation. Indications of undergauge hole stuck pipe mechanism are decrease in hook load in
a sudden, slightly/full unrestricted circulation and bit could not reach the bottom of the well or the previous zone after coring operation.
(Marbun et al., 2011)
5.1. Prevention of Stuck Pipe Due to Undergauge Hole
1. Identify the abrasive formation (quartzite, granite, etc)
2. Selects bit with good gauge protection
3. Never force a new bit to bottom
4. Ream the hole regularly
5.2 Mitigation of Stuck Pipe Due to Undergauge Hole
1. Keep circulating in borehole
2. If the drill sting is stuck when moving up the drill string then do the jarring operation downward and giving torque
3. If the drill string is stuck when moving down the drill sting the do the jarring operation upward but do not giving torque
4. Jarring operation starts by giving low load (50,000 lbs) and increase the load gradually in an hour. When jarring downward, the
circulation should be stopped or reduced. On the other hand, when jarring upward pump pressure could help to release the drill string so the circulation is recommended to be increased
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6. TIGHT HOLE
Tight hole is a phenomena that happened in the wellbore that caused by shale or clay formation that leads to wellbore hole enlargement.
The wellbore wall, which has clay formation become weakened by adsorption of water into the clay of the wellbore rock. (Marbun et.al,
2011)
Clay minerals can be divided into two broad groups. They are hydrophilic clays, which readily and easily absorb water, such as
montmorillonite. The other group is hydrophobic clays, which not readily and easily absorb water, such as illite. Clay minerals have a
sandwich-like structure usually consisting of three layers. The alternate layers are of silica and alumina. A clay particle usually consists
of several sandwiches stacked together like a pack of cards. (Heriot Watt Drilling Engineering).
The main study of clay support is clay that has special characteristic that high reactivity. Reactivity of the clay formation is known as
chemical activity. Chemical activity of the shale can be resulted to mud diffuses and mud will reacted especially using water as the
drilling fluid, mud chemistry changes, and time changes. Consequently, shall swells and reacts. In fresh water, the clay layers will
absorb water. The chemical bonds which holding them together are weakened and the stack of layers disintegrates. (Heriot-Watt
Drilling Engineering)
As the results of the swelling phenomena, tight hole incident was happened. Tight hole can be observed at the borehole and surface
indicators. Indicators that are used to detect tight hole in the borehole are using MWD, LWD and wireline. ROP that was observed was
decreased and DWOB that observed was decreasing. Tight hole can be analyzed from caliper log, which indicates that hole tightens
with time or dissolves. Using GR log, clay formation that caused tight hole can identified by GAPI>40. UBI can also detect swelling
which leads to tight hole.
In summarize for detecting tight hole at the borehole condition, Table 5 will summarize below.
Table 5. Wellbore Instability indicator due to tight hole at borehole condition
At the surface, tight hole condition can be observed by some indicators. Pump pressure was increased because of the tight hole will
make it harder to pump the mud. Mud flow is decreasing because shale will interact with the drilling mud that cause mud weight and
solids is increasing due to shale swelling. Cuttings and cavings that observed at the surface has special characteristics, for instance soft
and Gumbo cutting. Hookload has large overpull at connections, and the surface torque is increasing.
Indicators at the surface will be summarized at the Table 6 below.
Table 6. Wellbore instability indicator due to tight hole at surface condition
For preventing and mitigating the tight hole phenomena, some drilling actions can suppress the instability whereas some actions has no
influence on instability or makes it worse. Drilling actions that can suppress the instability are infrequent wiper trips and use inhibitive
mud. Drilling actions that has no influence on instability or makes the wellbore instability worse are frequent wiper trips, decrease ROP,